U.S. Pat. No. 3,040,005 discloses a process for the reconditioning and melt extrusion of condensed polyamides to give products of increased molecular weight and improved physical properties. A combination of mechanical work and vacuum is used to remove the water produced in the polymerization reaction and thus allow the molecular weight to increase. The residence time needed in the high vacuum area is 1 to 10 minutes to allow the simultaneous evaporation and reaction. Use of nitrogen sweeping through this section of the extruder was found to be equivalent to using a vacuum; thus using nitrogen at 2 psig was equivalent to a strong vacuum of 23.3 in Hg (223 millibar). Color was better when nitrogen was used. Where nitrogen was used, large quantities were needed (0.36 to 1.68 kg/kg of polymer). The technology taught in U.S. Pat. No. 3,040,005 is effective at the scale described in the patent examples. However, at the higher capacities required for industrial facilities, the molecular weight is increased to a much lower extent. Evaporation of the product of the condensation reaction is less effective because the molecules of the evaporating species must diffuse through larger distances of the polymer melt. Therefore commercial applications of this technology have been limited.
U.S. Pat. No. 4,760,129 discloses a process for the preparation of highly viscous polyhexamethyleneadipamide which comprises the steps of: introducing a melt of polyhexamethyleneadipamide into an extruder having a plurality of degassing zones; polycondensing the polymer in the extruder in a plurality of serially arranged zones of alternatingly elevated (above atmospheric pressure) and reduced (below atmospheric pressure) pressure; supplying superheated steam to the zones of elevated pressure; drawing off input vapor and water of condensation in the reduced pressure zones; and raising the temperature of the melt at a uniform rate over the length of the extruder. Large volumes of stripping gas are required, typically 0.1 to 0.5 kg of steam per kg of polymer. It is stated that the steam is needed to create foam and increase the amount of surface area for evaporation of the moisture produced by the reaction. Process temperature and vacuum levels are increased from port to port and molecular weight is increased. The patent states that no fewer than two stages are required. The disadvantages of the technology taught in U.S. Pat. No. 4,760,129 are the large quantities of steam required, and the complexity of the extruder required, since multiple evaporation ports are needed.
U.S. Pat. No. 5,236,645 discloses a process for introducing additives into a thermoplastic melt comprising the steps of: a) feeding at least one additive in an aqueous vehicle containing a dispersant to form an aqueous additive stream to a vented extruder which is extruding a thermoplastic; b) volatilizing the aqueous portion of the aqueous additive stream; c) removing substantially all the volatilized aqueous portion through the extruder vent to achieve a substantially homogeneous system containing the thermoplastic and at least one additive; and d) forming a fiber from the homogeneous system by extrusion of the homogeneous system through a spinneret. U.S. Pat. No. 5,236,645 describes a method of adding additives to a polymer melt but does not address a method for increasing the molecular weight of the polymer.
A practical limitation exists for all technologies where vent ports are used on condensation polymers. It is difficult to maintain a vacuum for long periods because a small amount of low molecular weight polymer evaporates in the vacuum area, settles or condenses on the equipment walls causing degraded polymer and eventually plugs the gas passage or falls down into the melt contaminating the melt. The existing art does not address these problems.
Before manufacture of moldings, extrusions or fibers, polyamide prepolymer melts are conventionally subjected to heat and vacuum in a variety of processing vessels in order to increase the molecular weight, or relative viscosity (RV) of the polymer to one that gives the appropriate physical properties and processing characteristics to the polymer. Alternatively, the polymer in pellet form is "solid phase" polymerized by passing warm dry gas over the polymer. Conventional melt polymerization processes might subject the polymer to heat and vacuum for a number of minutes, while solid phase polymerization might take many hours. In these cases the rate at which the polymer molecular weight is increased is limited by the rate at which water vapor can be removed from the melt or the solid particle. Commercially the polymer is often melt polymerized in autoclaves or continuous polymerization units to an RV of about 43, this being the molecular weight that is readily obtained in atmospheric pressure vessels. If higher melt viscosities are needed such as for improved spinning characteristics, the polymer is usually increased in molecular weight either by subjecting it in melt form to vacuum, or by solid phase polymerization as described above.
Another problem inherent in the prior art is the need to deal with moisture absorbed by the polymer during storage or shipment and prior to increasing molecular weight. For example, in solid phase polymerization, care must be taken to control moisture in the stripping gas and a significant part of the process time is simply used to remove moisture that might have been absorbed by the polymer, since polyamide absorbs moisture very easily. The present invention avoids the need for a separate drying step.
In the laboratory it is often possible to obtain significant increases in molecular weight or RV by the application of vacuum, agitation and gas sweeps. However, when rates are increased to industrial scales, residence times in vacuum zones are decreased, and molecular weight increases are less dramatic. Thus, there is a need for a process that reduces the time the polymer must spend in the equipment and thus improves quality and reduces expense, even at industrial rates. The present invention teaches such a process.
The prior art processes can be summarized in the following way. During condensation polymerization, the degree to which monomers are polymerized, and thus the molecular weight of the product, is limited by the laws of chemical mass action. This limit can be described in terms of an equilibrium between end groups on the polymer chains which produces longer polymer chains while the backward reaction is between dissolved by-product and amide or ester (in the case of polyamide and polyester, respectively) groups in the polymer which produces shorter chains. In the case of nylon 66, the end groups are amine and carboxyl groups, the by-product is water and the hydrolyzable groups are amide groups in the chain. The backwards reaction is often called the hydrolysis reaction. In order to promote the forward reaction in nylon and obtain high molecular weight, the byproduct water must be removed.
Based on these principles, it is generally believed that condensation polymerization requires the careful removal of any absorbed moisture, followed by the continuous removal of by-products during the course of the polymerization. For this reason, prior art processes have concentrated first on removing any absorbed moisture, followed by a lengthy removal of by-products. In the case of polyamides, after the polymer is dried, water formed by the polymerization is removed, in vapor form, and this is done by holding the melt under a vacuum for a significant period or in multiple stages in order to keep the polymerization moving forward. Normally, an effort is made to generate surface area in the polymer melt for water evaporation and to do this for long enough periods for the reaction to continue. U.S. Pat. No. 3,040,005 and U.S. Pat. No. 4,760,129 follow this procedure. The high residence time during evaporation required by the known processes is one reason why the technology is difficult to use on an industrial scale, since large expensive equipment is needed to maintain the vacuum for extended periods. High vacuum and or high volumes of sweep gas are also required, thus increasing the expense. The process of the present invention demonstrates that molecular weight can be increased by rapidly removing the dissolved by-products of condensation polymerization, such as water, providing that suitable devolatilization techniques are used and that the polymer melt is allowed suitable residence time after this stage. In the process of the present invention, the bulk of the reaction is done after the stripping step. In prior art processes the by-product was removed continuously or in multiple stages simutaneous with the reactions. The process taught herein produces high yields in standard equipment capable of commercial operation. Since the residence time under vacuum is short, the equipment needs are relatively inexpensive.